Teach ReMaterialization to be more cunning about subregisters

This allows rematerialization during register coalescing to handle
more cases involving operations like SUBREG_TO_REG which might need to
be rematerialized using sub-register indices.

For example, code like:
    v1(GPR64):sub_32 = MOVZ something
    v2(GPR64) = COPY v1(GPR64)
should be convertable to:
    v2(GPR64):sub_32 = MOVZ something

but previously we just gave up in places like this